feat(editor): metadata panel, subject editing, and variable catalogue

[patoperpetua]

Summary

• Adds MetadataPanel (key, name, description, subject; read-only schemaVersion) and VariableCatalogue (copy/insert, declare add/remove) to @singleton-sd/post-kit-editor.
• Wires both into EmailTemplateEditor working TemplateSourceFiles state beside the existing EmailBuilder canvas; subject supports cursor insert + live preview-data hint.
• Constrains template key input to publisher-safe charset ([a-zA-Z0-9._-], not ./..); clipboard copy degrades without throwing.

Closes #44

Test plan

• pnpm --filter @singleton-sd/post-kit-editor test (45 passing)
• pnpm --filter @singleton-sd/post-kit-editor build
• Confirm canvas from EmailBuilder.js canvas and lossless template source load/serialize #40 still renders editable document in a consumer host
• Smoke: edit name/key/subject; reject key with / or spaces; insert {{name}} into focused subject; copy placeholder

Made with Cursor

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Configuration used: Organization UI

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Run ID: 781358af-51f9-4c40-8b9a-6f7a5576201f

📥 Commits

Reviewing files that changed from the base of the PR and between e40f3e1 and ca8a002.

📒 Files selected for processing (15)

• packages/post-kit-editor/src/email-template-editor.tsx
• packages/post-kit-editor/src/index.spec.tsx
• packages/post-kit-editor/src/insertion-target.tsx
• packages/post-kit-editor/src/metadata/MetadataPanel.spec.tsx
• packages/post-kit-editor/src/metadata/MetadataPanel.tsx
• packages/post-kit-editor/src/metadata/insert-at-cursor.ts
• packages/post-kit-editor/src/metadata/metadata-helpers.spec.ts
• packages/post-kit-editor/src/metadata/subject-preview.ts
• packages/post-kit-editor/src/metadata/template-key.ts
• packages/post-kit-editor/src/types.ts
• packages/post-kit-editor/src/variables/VariableCatalogue.tsx
• packages/post-kit-editor/src/variables/clipboard.ts
• packages/post-kit-editor/src/variables/resolve-catalogue.ts
• packages/post-kit-editor/src/variables/variable-catalogue.spec.tsx
• packages/post-kit-editor/src/working-files.ts

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[patoperpetua]

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